This invention relates to a cylindrical coil which provides a magnetic field having a substantially linear gradient in an axial direction within the volume of the cylinder. More specifically, this invention relates to a cylindrical coil which provides a magnetic field having a substantially linear gradient in an axial direction within the volume of the cylinder so as to be useful for obtaining spatial information for nuclear magnetic resonance NMR imaging.
In NMR imaging, a homogeneous magnetic field B.sub.0 is directed along the positive z-axis of a Cartesian coordinate system whose origin is typically near the center of the sample to be imaged. The Z-axis is selected to be coincident with the longitudinal axis of a typically cylindrical sample volume. The effect of the static magnetic field B.sub.0 is to polarize nuclear spins having net magnetic moments so that at equilibrium a greater number of nuclear spins align with the B.sub.0 field and produce a macroscopic magnetization M. This polarization allows a resonant response of the nuclear spins when excited by radio frequency (RF) magnetic field pulses. When RF pulses of appropriate frequency and direction are applied to the sample, nuclei of the sample are excited by the RF energy and individual polarized nuclear spins (and hence magnetization M) precess about the axis of the field B.sub.0, at a frequency .omega. given by the equation .omega.=.gamma.B.sub.0, in which .gamma. is the gyromagnetic ratio for the isotope or isotopes being studied. The absorbed energy is detected as an NMR signal in the form of an RF magnetic field resulting from the precession of the nuclei.
Magnetic field gradients are used in NMR imaging systems to encode spatial information into the NMR signal. Typically three orthogonal magnetic field gradients are used, corresponding to the three axes of the Cartesian coordinate system. If the strength of the component of a magnetic field along the direction of one of those axes is a function of position within an imaging volume, then so is the resonant frequency of the nuclear spins.
If the magnetic field gradient is linear, the frequency spectrum for the spins is a one-dimensional projection of the NMR signal distribution along the direction of the field gradient. Thus, each value of the magnetic field corresponds to one location along the axis of the field. Accordingly, if the value of the magnetic field is the same at two or more different locations along the axis, the NMR signals for those locations are combined, resulting in degradation of the NMR signal data and image artifacts. Similarly, magnetic field gradients which are non-linear can cause geometric distortions of the image. Thus, magnetic field gradients which are highly linear are desirable.
More complete treatments of basic NMR concepts are provided in a recent text edited by Leon Kaufman et al., entitled "Nuclear Magnetic Resonance Imaging and Medicine", Igaku-Shoin, New York in Tokyo (1981), and also in an earlier text by Thomas C. Farrar et al., entitled "Pulse and Fourier Transform NMR, An Introduction to Theory and Methods", Academic Press, New York (1971), which are incorporated herein by reference as background material.
In NMR whole-body imaging it is convenient to wind the coils for the magnetic field gradients on a cylindrical surface, because the openings at the ends of the cylinder provide a means of access through which the patient can be introduced into the imaging volume. In such a system, the central axis of the cylinder is aligned with the Z-axis of the imaging system referred to above. Thus, a coil design is desired that produces a magnetic field B.sub.z that is linearly graded along the Z-axis, that is, a field in the Z-direction of the form B.sub.z =G.sub.z Z, where G.sub.z, the field gradient, is a constant. Different coil designs and magnetic fields may be effected by selecting different surface current density patterns on the cylindrical coil surface. A number of coil arrangements have been proposed to approximate a linearly graded magnetic field, but they all produce magnetic field gradients which contain non-linear terms that lead to both axial and radial distortions in the field. For example, a coil made from Maxwell pairs has been used in the past to provide magnetic field gradients. While such coils are capable of providing a nearly linear field gradient at the center of the cylinder, the field gradient becomes non-linear at distances from the center which are more than approximately one-half the radius of the cylinder. It is not practical merely to make the coil larger in an attempt to increase the area of linearity because, for whole-body imaging, it is already very large in terms of the magnet system needed to produce the high B.sub.0 field required for NMR imaging.
Another problem with coils that have been used in the past to provide magnetic field gradients is the relatively high electrical inductance of such coils. The pulse sequences used in NMR imaging require coils that are capable of being rapidly switched between high current and low current conditions. Therefore, coils having low electrical inductance are desired.
It is an object of the present invention to provide a cylindrical coil having a substantially linearly graded magnetic field within the interior volume thereof.
It is a further object of the present invention to provide a magnetic field gradient coil having low electrical inductance.
It is also an object of the present invention to provide a cylindrical magnetic field gradient coil especially suitable for use with NMR apparatus.